1. Field of the Invention
The present invention relates to an oxygen concentration detecting apparatus having a limit current type oxygen sensor comprising an oxygen concentration detecting element that outputs limit current proportional to oxygen concentration and a heater for heating the detecting element and, more particularly, to an oxygen concentration detecting apparatus that checks for abnormality of the limit current type oxygen sensor.
2. Description of Related Art
Many modern air-fuel ratio control systems use limit current type oxygen sensors (oxygen concentration detectors). In such a system, the oxygen concentration detected by the air-fuel ratio sensor is inputted to a microcomputer to calculate an air-fuel ratio, and the microcomputer performs air-fuel ratio feedback control based on the calculated air-fuel ratio. The control system thereby achieves optimal combustion in the internal combustion engine and reduces harmful substances in exhaust gas, such as CO, HC, NOx and the like.
However, since the control precision of the air-fuel ratio control systems is heavily degraded if the reliability of detection of the air-fuel ratio deteriorates, there has been a strong demand for a technology that precisely detects an abnormality of an air-fuel ratio sensor. For example, Japanese Unexamined Patent Application Publication No. Hei. 1-232143, "Air-Fuel Ratio Control Apparatus for Internal Combustion Engine", describes a technology that detects an abnormality of a heater if the temperature of the air-fuel ratio sensor (oxygen concentration detecting element) detected by a temperature sensor fails to rise to a predetermined temperature. Japanese Unexamined Patent Application Publication No. Hei. 3-189350, "Oxygen Sensor Heater Control Apparatus", describes a technology for use in an apparatus for controlling the power supply to the heater so that the heater resistance becomes equal to a target resistance, the technology detecting an abnormality of the target resistance if the power supply to the heater deviates from a predetermined range.
However, the conventional art has the following problems. The aforementioned former technology (Japanese Unexamined Patent Application Publication No. Hei. 1-232143) requires a sensor for detecting the temperature of the air-fuel ratio sensor, and thus has problems of high costs. The latter technology (Japanese Unexamined Patent Application Publication No. Hei. 3-189350) merely determines whether the target resistance is properly set, and the occasions when this diagnosis technology detects abnormality are substantially limited to the occasions when the battery or the sensor has been replaced. Thus, this technology does not make a determination regarding the reliability of the oxygen sensor.